Delayed tail fin deployment mechanism and method

ABSTRACT

A hold down device positioned on the projectile to exert a known spring force in opposition to the centrifugal force provides an inexpensive, light weight and reliable delayed fin deployment mechanism for boosted fin-stabilized spinning projectiles. When the forcing moment produced by the centrifugal force acting on the fin exceeds the opposing moment produced by the hold down device, the hold down device will release the fin allowing it to swing into its deployed position. Thus, proper selection of the spring force and positioning of the hold down device will cause the fins to deploy at a predetermined spin rate. The spin rate can be correlated to a time or travel distance of the projectile from launch. The incorporation of the hold down devices requires minimal design changes to existing rockets and may, in some cases, be retrofit to the existing base of rockets if desired.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to fin-stabilized projectiles and moreparticularly to a mechanism for delayed tail fin deployment.

2. Description of the Related Art

Modern warfare is based on mission speed, high per round lethality, andlow possibility of collateral damage. This requires that the ordinancebe delivered on target with high precision. An important component toachieving high precision is to maintain the stability of the projectiledelivering the ordinance. High spin rate projectiles such as bullets,artillery shells or ballistic missiles are self-stabilizing(“spin-stabilized”), the projectile acts like a gyro which prevents theprojectile from tumbling. Low spin rate projectiles such as rockets(guided or unguided) deploy tail fins to shift the center of pressureaft of the center of gravity to ensure stability (“fin-stabilized”).Roll-stabilized projectiles such as guided missiles use active controlof tail fins and other aerodynamic surfaces to provide stabilization.

An exemplary weapon system 10 is illustrated in FIGS. 1, 2 and 3 a-3 b.In this example, the weapon system is a multi-tube rocket launcher 11mounted on a helicopter 12 that fires rockets 13. Tail fins 14 arestowed in a spring-loaded overlapping (FIG. 3 a) or wrap-around designaround the circumference of rocket tail section 15 while inside the tube16. The tail section also includes a nozzle 17 and rocket motor (notshown) to provide boost. To provide some stability the rocket nozzlesare scarfed at an angle to impart a slight spin to the rocket duringflight, e.g. 20-60 cycles/second typically. Alternately, vanes could bepositioned aft of the nozzle to impart the spin. The tail section 15 iscoupled to the main body 18 of the projectile on which a warhead 19 andfuze 20 are attached. As shown, rockets 13 are unguided, simply pointand shoot. A guidance package could be inserted between the warhead andmain body in which case additional canards would be controlled to guidethe rocket based on, for example, GPS or sensor data. Also, individualrockets may be launched from a pylon instead of a tube.

As shown in FIG. 3 a, as the rocket spins up in the launch tube 16 acentrifugal force 24 is generated that produces a rotational moment onthe fins about their respective rotation pins 26. Once clear of thetube, absent some additional restraint, centrifugal force 24 willimmediately rotate the fins to their deployed positions as shown in FIG.3 b. Spring loading adds to the centrifugal force to deploy the finsmore quickly and with less variation. This “passive-passive” system e.g.passive deployment and passive control, is inexpensive, lightweight, lowvolume and reliable. The fins, once deployed, are typically held inposition by a locking mechanism. Deployment is immediate upon clearingthe launch tube. There is no capability to delay or control findeployment to, for example, avoid interference with adjacent rockets orto mitigate the effects of boost-phase winds associated with, forexample, the flow field of the helicopter.

D. J. Wilson “Delayed Fin Deployment Mechanism” (Lockheed-HuntsvilleResearch and Engineering Center, Huntsville Ala. 1978) describes an“active-passive” system that provides for delayed deployment but atsignificantly higher cost, weight, and volume. A timing circuit fires abridge wire activated cable cutter squib after a precise time delayinitiated by the rocket ignition pulse. The squib, in turn, clips andthus releases a stainless steel cable which had previously maintainedthe spring-loaded fins in a folded position. Each (of two) timercircuit/squib units with batteries is contained in a packageapproximately the size of a pack of cigarettes.

Some systems use the tail fins to provide both stability and guidancecontrol instead of using additional canards. These “active-active”systems are quite expensive and large as they must provide both theactuator mechanism to physically adjust the fins and the intelligence toproportionally control the actuator mechanism in Teal-time to guide therocket. The actuator mechanism may be mechanical, electromagnetic orpossibly electrostatic. This guidance capability is more than sufficientto delay deployment of the tail fins but at a high cost.

A need remains for a fin deployment mechanism having rudimentary timingcontrol that does not sacrifice cost, weight, volume or reliability.Ideally, such a fin deployment mechanism should require minimal redesignof existing rockets with the potential to retrofit the existinginventory of rockets.

SUMMARY OF THE INVENTION

The present invention provides an inexpensive, light weight, low volumeand reliable delayed fin deployment mechanism for boosted fin-stabilizedspinning projectiles.

This is accomplished with a hold down device that holds the fin in itsstowed position with a constant spring force. During the boost stage,the projectile spins up to its terminal spin rate. The spring force isselected to correspond to a particular spin rate of the projectile (lessthan the terminal spin rate), which in turn is correlated to a desiredtravel distance of the projectile from launch. When the spin ratereaches the target value the rotational moment produced by thecentrifugal force exceeds the opposing moment produced by the springforce and the hold down device releases the fin to pivot outwardly toits deployed position. The hold down device provides a very simple andreliable solution to allow a boosted spinning projectile to, forexample, clear an aircraft's flow field and/or other projectiles in amulti-tube launcher.

A typical projectile will include a plurality of fins positioned aroundthe circumference of the projectile's tail section. In one embodiment,each fin will be provided with a hold down device. Ideally each devicewill exhibit the same spring force so that all of the fins deploy at thesame time. However, inevitably there is some variation in the springforces that causes a degree of dispersion at the target. In anotherembodiment, a plurality of cams are positioned between adjacent fins sothat when the hold down device having the weakest spring force releases,the deployment of its fin pushes the cam against the adjacent fincausing its hold down device to release and so forth in a daisy chainuntil all of the hold down devices have been released and the finsdeployed. The cams should reduce dispersion at the target. In yetanother embodiment, only a primary fin is held in place with a hold downdevice. The remaining secondary fins are captured by a lanyard that isheld between a pair of attachment lugs. The deployment of the primaryfin releases the lanyard from at least one of the attachment lugsthereby allowing the secondary fins to deploy almost simultaneously.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, as described above, is a diagram of a multi-tube rocket launchermounted on a helicopter;

FIG. 2, as described above, is a diagram of a fin-stabilized rocket;

FIGS. 3 a-3 b, as described above, are section views of the spinningrocket illustrating the centrifugal forces on the stowed fins in or outof the launch tube and the fins in their deployed positions post launchout of the launch tube;

FIG. 4 is a section view of the spinning projectile illustrating a holddown spring force that opposes the centrifugal force to delay deploymentof the fins in accordance with the present invention;

FIGS. 5 a-5 b are plots of the forcing moment and travel as the boostedprojectile spins up, respectively;

FIG. 6 is a perspective view of a multiple spring-cam fin deploymentmechanism;

FIG. 7 is a perspective view of an exemplary hold down device;

FIG. 8 is a section view of the deployment mechanism illustrating thedaisy chain effect when the first fin is released;

FIG. 9 is a perspective view of a single spring-lanyard fin deploymentmechanism;

FIG. 10 is a section view of the deployment mechanism illustrating therelease of the lanyard to deploy all of the fins;

FIG. 11 is a view of an alternate embodiment of the singlespring-lanyard fin deployment mechanism in which the fins are stowed ina jack-knife configuration inside the tail section; and

FIG. 12 is a diagram illustrating deployment of the primary fin therebyreleasing the lanyard from the master lug.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an inexpensive, light weight and reliabledelayed fin deployment mechanism for boosted fin-stabilized spinningprojectiles. A hold down device is positioned on the projectile to exerta known spring force in opposition to the centrifugal force. When theprojectile is launched it is boosted and spins up to a terminal spinrate. The centrifugal force increases with the square of the spin rate.When the moment produced by the centrifugal force acting on the finexceeds the opposing moment produced by the hold down device, the holddown device will release the fin allowing it to swing into its deployedposition. Thus, proper selection of the spring force and positioning ofthe hold down device will cause the fins to deploy at a predeterminedspin rate. The spin rate can be correlated to a time or travel distanceof the projectile from launch. Thus, the hold down device(s) provides asimple yet effective means for delayed fin deployment in a boostedfin-stabilized spinning projectile. The incorporation of the hold downdevices requires minimal design changes to existing rockets and may, insome cases, be retrofit to the existing base of rockets if desired.

As shown in FIGS. 4 and 5 a-5 b, a hold down device or devices 50 arepositioned around the circumference of projectile 13 to restrain fins 14in their stowed position as the projectile spins 52 around its axis 54.The hold down device exerts a constant spring force 56 on the fin thatopposes centrifugal force 24. Centrifugal force 24 is given byF_(c)=m*s^(2*)r lb where m is the mass of the projectile, r is theradius from the spin axis to the fin center of mass and s is the spinrate. The centrifugal force acting through the center of mass of the finproduces a moment M_(C)=d_(F)*F_(C) where d_(F) is the distance from finrotation pin 26 to the center of mass of the fin. Spring force 56 isdetermined by the design of a particular hold-down device 50. Theopposing moment M_(s)=d_(s)*F_(S) where d_(s) is he distance from finrotation pin 26 to hold-down device 50 and F_(S) is the spring force.Thus, the forcing moment M_(C) is dictated by projectile and fin designand by the boost. The opposing moment M_(S) is set through a combinationof the spring force and the placement of the hold-down device.

As shown in FIG. 5 a, in a “boosted” projectile the spin rate, hencecentrifugal force and moment M_(C) spins up from zero to a terminal ormaximum value 60 during the boost phase 62. The projectile, as shown inFIG. 2, includes a rocket motor and nozzle that propels the projectiletowards the target and induces spin such as found in surface-to-air orair-to-air rockets and missiles. The boost phase of a typical rocket is,for example, 1 to 0 seconds in duration during which time the spin rate,hence centrifugal force is increasing. Thus, the boost phase 62 definesa time window from to at launch to t_(terminal) at the end of the boostphase in which to delay the deployment of the tail fins. Hold-downdevice 50 is designed and positioned to produce an opposing moment M_(S)that lies somewhere above the minimum moment M_(C)=0 and somewhere belowthe maximum moment at the terminal spin rate. The tail fins will deployat a time t₁ when moment M_(C) exceeds the opposing moment M_(S).

As shown in FIG. 5 b, the travel 70 of the projectile can be accuratelyplotted against time for a given projectile design and boost. Tail findeployment can be delayed to correspond to a desired travel distance ofthe projectile up to a maximum travel delay d_(max) corresponding to theend of the boost phase. Once boost is completed, the spin rate, hencemoment M_(C) will not get any larger and will actually reduce slightlydue to aerodynamic drag effects. Assuming a battlefield scenariorequires the projectile to travel at least a distance d_(min) before thefins are deployed, a designer might select a distanced_(min)<d₁<d_(max). How close the designer sets d₁ to d_(min) may dependon a number of considerations including the manufacturing tolerance ofthe actual spring force to the design value, the accuracy with whichtravel is known as a function of time for a particular projectile andboost, the criticality of not deploying the fins early and converselythe criticality of not deploying the fins too late. The selection of d₁determines the time of deployment t₁, which in turn determines theopposing moment M_(S). The design can than select the spring force ofthe hold-down device and the position of the hold-down device to achievethe required moment.

The hold down device provides a very simple and reliable solution toallow a spinning projectile to, for example, clear an aircraft's flowfield and/or other projectiles in a multi-tube launcher. In bothinstances, the travel delay can be established a priori based onknowledge of the aircraft or the multi-tube launcher. For example, adesigner can estimate that for a certain type of helicopter whenhovering to fire its rockets the flow field produced by the rotors couldcause the rocket to turn into the flow field and away from the intendedtarget if the tail fins were deployed within 10 meters of thehelicopter. Assuming that the boost phase extends beyond 10 meters, thedesigner can select and position a simple hold-down device to delay tailfin deployment. In the multi-tube launcher application, if the tail finsdeploy immediately upon clearing the tube they can interfere withadjacent rockets extending from their tubes. In this case, the traveldelay need only be sufficient for the rocket to clear the other rockets.Note, if a longer travel delay is required, it may be possible to extendthe boost phase.

A typical projectile will include a plurality of fins positioned aroundthe circumference of the projectile's tail section. The fins may be flator curved to wrap-around the projectile. Alternately, the fins may bejack-knifed inside the tail section. In one embodiment, each fin will beprovided with a hold down device (FIGS. 6-8). Ideally each device willexhibit the same spring force so that all of the fins deploy at the sametime. However, inevitably there is some variation in the spring forcesthat causes a degree of dispersion at the target. In another embodiment,a plurality of cams are positioned between adjacent fins so that whenthe hold down device having the weakest spring force releases, thedeployment of its fin pushes the cam against the adjacent fin causingits hold down device to release and so forth in a daisy chain until allof the hold down devices have been released and the fins deployed (alsoFIGS. 6-8). The cams should reduce dispersion at the target. In yetanother embodiment, only a primary fin is held in place with a hold downdevice. The remaining secondary fins are captured by a lanyard that isheld between a pair of attachment lugs. The deployment of the primaryfin releases the lanyard from at least one of the attachment lugsthereby allowing the secondary fins to deploy almost simultaneously(FIGS. 9-10). The single lanyard mechanism can also be adapted for usewith the jack-knife fin configuration (FIGS. 11-12).

As shown in FIG. 6-8, a plurality of fins 80 are positioned around thecircumference of the nozzle (not shown) and pivotally mounted along aninterior longitudinal edge 82 on respective fin rotation pins 84extending through fin hubs 85 along a main axis 86 of the projectile toswing from a stowed position against the nozzle to a deployed position.A like plurality of hold down devices 88 are positioned to hold the finsin their stowed positions. In this particularly embodiment, each holddown device 88 (best shown in FIG. 7) is positioned on the fin rotationpin 84 of the adjacent fin to hold the lateral edge 90 of the fin nearits exterior longitudinal edge 92.

The hold down device is configured to provide a predetermined springforce opposing the deployment of the fin until the forcing moment issufficiently large to overcome the spring force and push the hold downdevice out of the way. The spring force is determined by length, width,thickness, shape and material composition of walls 94 and can be definedand manufactured to a reasonable tolerance. Friction between the fin andhold down device has considerably more variation as it depends upon suchunknowns as dirt, humidity etc. Consequently, it is generally desirableto design the hold down device (shape) to minimize friction. In thisparticular embodiment, the edge 96 of the hold down device that actuallycontacts the fin is rounded to minimize any friction between the fin anddevice as the fin pushes edge 96 outward from the projectile spin axis86 during deployment. The rounded edge also reduces the likelihood thatthe edge will tear or otherwise damage the fin during deployment.

Ideally each hold down device 88 will exhibit the same spring force sothat all of the fins deploy at the same time. However, inevitably thereis some variation in the spring forces that causes a degree ofdispersion at the target. To reduce dispersion, a like plurality of cams98 are positioned between adjacent fins 82 so that when the hold downdevice 88 having the weakest spring force releases, the deployment ofits fin 80 pushes the cam 98 against the adjacent fin causing its holddown device to release and so forth in a daisy chain until all of thehold down devices have been released and the fins deployed. In thisparticular fin configuration, the cams 98 are positioned axially betweenthe interior longitudinal edge 82 of one fin and the exteriorlongitudinal edge 92 of the adjacent fin so that when the hold downdevice having the weakest spring force releases the deployment of itsfin pushes the cam against the exterior longitudinal edge of theadjacent fin causing its hold down device to release and so forth in thedaisy chain. The force exerted by the cams should be larger than anyvariance in the spring forces of the hold down devices. For the typicalcase in which all of the hold down devices are designed to have the samespring force, any one of the hold down devices may be the weakest andstart the daisy chain. Alternately, a fin could be designated as theprimary fin and its hold down device designed specifically to have theweakest spring force. The remaining secondary fins would have a higherdesigned spring force. When the primary hold down device releases, itstarts the daisy chain and the cams provide sufficient additional forceto deploy the secondary fins.

Although not shown, a typical deployment mechanism may also include aspring underneath each fin to more rapidly deploy the fin once released.If the spring assist is included the spring force of the hold downdevice is increased to offset the spring assist so that the tail finsdeploy at the same delay. The only effect is that once the fins arereleased, the forcing moment includes both the centrifugal force and thespring assist so that the fin will deploy faster. A typical deploymentmechanism may also include a fin locking mechanism on the fin hub thatholds the fin its deployed position. The centrifugal force of thespinning projectile will tend to hold the fin in the deployed positionbut the locking mechanism provides an additional measure of stabilityand reliability. The locking mechanism can be a simple detent.

In an alternate embodiment shown in FIGS. 9 and 10, a single hold downdevice 100 is positioned to hold a primary fin 102 against the nozzle104 in the tail section of the projectile. A lanyard 106 is securedbetween primary and secondary attachment lugs 108 and 110, respectively,around the projectile to restrain one or more secondary fins 112 intheir stowed positions. The deployment of primary fin 102 releases thelanyard 106 from first attachment lug 108 thereby allowing the secondaryfins 112 to deploy. Primary attachment lug 108 is suitably positioned onthe primary fin 102 and preferably on the fin rotation hub 114 so thatas the fin pushes (deploys) past the hold down device 100 to rotate intoits deployed position, the primary lug 108 also rotates allowing thelanyard to slip off. The secondary attachment lug 110 is positionedelsewhere on the projectile, suitably on the rotation hub 114 of thelast secondary fin 112. When the lanyard slips off, the centrifugalforce pops open all of the secondary fins almost simultaneously. Thespring assist and locking mechanism may also be used in thisconfiguration.

In an alternate embodiment shown in FIGS. 1 and 12, a single hold downdevice 200 and lanyard 202 are used to hold a plurality of fins in ajack-knifed configuration. U.S. Pat. Nos. 6,764,042 and 6,588,700describe a tactical base for a guided projectile in which the fins arestored in a jack-knife configuration, which are hereby incorporated byreference. The projectile's tail section 204 can be similarlyreconfigured by forming a plurality of conical sections 208 spacedaround the nozzle 206 to define fin slots 210. Fins 212 are pivotablymounted on fin pins 214 within the fin slots in a stowed position. Thehold down device 200 is positioned over one of the fin slots at adetermined distance from the fin pin (measured along the longitudinalaxis of the projectile), The primary lug 216 is positioned on the holddown device so that when the forcing moment of the centrifugal forceexceeds the opposing moment of the hold down device the fin pushes pastthe hold down device causing primary lug 216 to rotate and releaselanyard 202. The secondary lug 218 is suitably position on the conicalsection 208 past the last fin.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. A delayed tail fin deployment mechanism, comprising: a projectilehaving an engine and nozzle configured to spin up the projectile duringa boost phase following launch; a fin that is pivotally mounted on theprojectile, said fin being stowed at launch so that the centrifugalforce of the spinning projectile produces a moment that rotates the fininto a deployed position; and a hold down device that holds the fin inits stowed position until the moment of centrifugal force exceeds anopposing moment produced by a spring force of the hold down device, saidspring force being predetermined to correspond to a particular spin rateof the projectile.
 2. The fin deployment mechanism of claim 1, whereinthe particular spin rate of the projectile is correlated to the traveldistance of the projectile from launch.
 3. The fin deployment mechanismof claim 1, further comprising a plurality of said fins positionedaround the projectile and a like plurality of said hold down devicesthat hold respective fins in their stowed positions.
 4. The findeployment mechanism of claim 3, wherein all of said hold down devicesare designed to release at the same spin rate.
 5. The fin deploymentmechanism of claim 4, wherein the spring force of said hold down deviceswill have some amount of variability, further comprising a plurality ofcams positioned between adjacent fins so that when the hold down devicehaving the weakest spring force releases the deployment of its finpushes the cam against the adjacent fin causing its hold down device torelease and so forth in a daisy chain until all of the hold down deviceshave been released and the fins deployed.
 6. The fin deploymentmechanism of claim 5, wherein each said fin has an interior longitudinaledge that is pivotally mounted along a main axis of the projectile andan exterior longitudinal edge, said cams are positioned axially betweenthe interior longitudinal edge of one fin and the exterior longitudinaledge of the adjacent fin so that when the hold down device having theweakest spring force releases the deployment of its fin pushes the camagainst the exterior longitudinal edge of the adjacent fin causing itshold down device to release and so forth in the daisy chain.
 7. The findeployment mechanism of claim 1, further comprising a primary fin and aplurality of secondary fins positioned around the projectile, said holddown device holding the primary fin in the stowed position, furthercomprising: a first attachment lug; a second attachment lug; and alanyard between the first and second attachment lugs around saidprojectile that restrains the secondary fins in their stowed positions,wherein the deployment of the primary fin releases the lanyard from saidfirst attachment lug thereby allowing the secondary fins to deploy. 8.The fin deployment mechanism of claim 7, wherein the first attachmentlug is positioned on the primary fin and the second attachment lug ispositioned elsewhere on the projectile.
 9. The fin deployment mechanismof claim 8, wherein each said fin has an interior longitudinal edge thatis pivotally mounted on a fin rotation hub along a main axis of theprojectile and an exterior longitudinal edge, wherein the firstattachment lug is positioned on the primary fin's fin rotation hub andthe second attachment lug is positioned on the secondary fin's finrotation hub immediately adjacent to the exterior longitudinal edge ofthe primary fin.
 10. The fin deployment mechanism of claim 9, where thefirst attachment lug is configured so that the lanyard slips off whenthe primary fin's fin rotation hub rotates.
 11. The fin deploymentmechanism of claim 7, wherein the first attachment lug is positioned onthe hold down device.
 12. The fin deployment mechanism of claim 11,wherein the plurality of fins are stowed in a jack-knife configurationinside the projectile.
 13. A delayed fin deployment mechanism for aweapon system, comprising: a multi-tube rocket launcher, a plurality ofrockets in and extending out from said tubes, each said rocketincluding: a rocket engine and nozzle configured to propel and spin upthe rocket during a boost phase following launch; a fin that ispivotally mounted on the projectile, said fin being stowed at launch sothat the centrifugal force of the spinning projectile produces a forcingmoment that rotates the fin into a deployed position; and a hold downdevice that holds the fin in its stowed position until the forcingmoment exceeds an opposing moment produced by a spring force of the holddown device, said spring force being predetermined to correspond to aparticular spin rate of the projectile that is correlated to a traveldistance of the projectile selected to clear adjacent rockets before thefins deploy.
 14. The weapon system of claim 13, further comprising aplurality of said fins positioned around the rocket and a like pluralityof said hold down devices that hold respective fins in their stowedpositions.
 15. The weapon system of claim 14, wherein the spring forceof said hold down devices have some amount of variability, furthercomprising a plurality of cams positioned between adjacent fins so thatwhen the hold down device having the weakest spring force releases thedeployment of its fin pushes the cam against the adjacent fin causingits hold down device to release and so forth in a daisy chain until allof the hold down devices have been released and the fins deployed. 16.The weapon system of claim 13, further comprising a primary fin and aplurality of secondary fins positioned around the rocket, said hold downdevice holding the primary fin in the stowed position, furthercomprising: a first attachment lug; a second attachment lug; and alanyard between the first and second attachment lugs around said rocketthat restrains the secondary fins in their stowed positions, wherein thedeployment of the primary fin releases the lanyard from said firstattachment lug thereby allowing the secondary fins to deploy.
 17. Amethod for delayed deployment of tail fins on a boosted fin-stabilizedspinning projectile, comprising: passively applying a spring force tohold a fin in its stowed position, said spring force corresponding to aparticular spin rate of the projectile; boosting the projectile over aboost phase to propel the projectile towards a target; manipulating theboost to spin up the projectile; and passively releasing the fin to adeployed position when the centrifugal force of the spinning projectileproduces a forcing moment that exceeds an opposing moment produced bythe spring force.
 18. The method of claim 17, further comprising:correlating the particular spin rate at which the fins deploy to adesired travel distance.
 19. The method of claim 17, whereinapproximately the same spring force is applied to each of a plurality offins positioned around the rocket so that when the fin having theweakest applied spring force deploys that fin interferes with theadjacent fin causing the adjacent fin to deploy and so forth in a daisychain until all of the fins have been deployed.
 20. The method of claim17, wherein the spring force is applied to a single primary fin, furthercomprising looping a lanyard between first and second attachment lugsaround said projectile to restrain a plurality of secondary fins intheir stowed positions, whereby the deployment of the primary finreleases the lanyard from said first attachment lug thereby allowing thesecondary fins to deploy.